The invention relates to an electrical machine having 12 stator teeth and 14 rotor poles, in particular for use in electrical steering aids.
In the case of electrical drives for steering systems with electromechanical assistance for use in motor vehicles, it is necessary for the fluctuations in the drive torque produced in the shaft to be as small as possible. Electrically commutated synchronous motors with permanent magnet excitation are normally used for applications such as these, because these motors have advantages owing to their power density, their efficiency and their control capability.
However, in the case of electronically commutated synchronous motors, harmonics result in so-called harmonic torques, which can lead to major oscillations in the torque. Drives such as these must therefore be designed to reduce these harmonics as much as possible or to keep their effects on the torque profile as small as possible. Furthermore, in the case of synchronous motors such as these, torque fluctuations occur not only when on load but also when no current is flowing the stator windings, and this is referred to as the cogging torque. Particularly in the case of small machines, it is impossible, because of the space conditions, to introduce a finely distributed winding into the armature, in order to produce an ideal sinusoidal air-gap field. In the case of small machines, corresponding harmonics therefore have to be expected in the air gap. It would therefore be advantageous to design the machines such that these harmonics have as little effect as possible on the torque.
It is also desirable to reduce the failure probability of an electrical machine such as this, and to keep the resultant braking torques comparatively low in the event of a fault, for example in the event of a short in the winding. These braking torques occur since, in contrast to machines with electrically excited fields, it is impossible to switch off the magnetic field in machines with permanent-magnet excitation. Electrical machines having 12 stator teeth and 8 rotor poles are already known, but these do not ensure adequate torque quality during normal fault-free operation.
Furthermore, an electrical machine having 18 stator teeth and 8 rotor poles is known, but this has the disadvantage that the electrical machine has a distributed winding in which the coil windings of different phases cross in the end winding, and coil sides of different phases are located in the stator slots. Shorts can therefore occur between different phases in the end windings and in the slot.
Furthermore, electrical machines having 12 stator teeth and 10 rotor poles are known, which allow smaller cogging torques in comparison to electrical machines having 12 stator teeth and 8 rotor poles, but produce a lower torque quality in comparison to an electrical machine having 18 stator teeth and 8 rotor poles.
Electrical machines having 12 stator teeth and 8 or 10 rotor poles generally have tooth heads in order to achieve small cogging torques. These make it more difficult to fit the coil windings to the stator teeth, and in general require split stators for a compact stator configuration, in order to allow a high conductor filling factor to be achieved for the coil windings. Split stators lead to asymmetries because of dimensional tolerances, as a result of which poor cogging torques can occur.